1. Statement of the Technical Field
The inventive arrangements relate generally microstrip patch antennas and more particularly to slot fed microstrip patch antennas.
2. Description of the Related Art
RF circuits, transmission lines and antenna elements are commonly manufactured on specially designed substrate boards. Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the dielectric constant.
For the purposes RF circuits, it is generally important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, signal reflections and inefficient power transfer can result. Electrical length of transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting circuit performance relate to the dielectric constant (sometimes referred to as the relative permittivity or ∈r) and the loss tangent (sometimes referred to as the dissipation factor) of the dielectric substrate material. The relative permittivity determines the speed of the signal in the substrate material, and therefore the electrical length of transmission lines and other components disposed on the substrate. The loss tangent determines the amount of loss that occurs for signals traversing the substrate material. Dielectric losses increase as the signal frequency increases. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar to microstrip except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes.
In general, the characteristic impedance of a parallel plate transmission line, such as stripline or microstrip, is approximately equal to √{square root over (Ll/C1)}, where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the physical geometry, the spacing of the line structure, as well as the permittivity and permeability of the dielectric material(s) used to separate the transmission lines.
In conventional RF designs, a substrate material is selected that has a single relative permittivity value and a single relative permeability value, the relative permeability value being about 1. Once the substrate material is selected, the line characteristic impedance value is generally exclusively set by controlling the geometry of the line.
Radio frequency (RF) circuits are typically embodied in hybrid circuits in which a plurality of active and passive circuit components are mounted and connected together on a surface of an electrically insulating board substrate, such as a ceramic substrate. The various components are generally interconnected by printed metallic conductors, such as copper, gold, or tantalum, which generally function as transmission lines (e.g. stripline or microstrip or twin-line) in the frequency ranges of interest.
The dielectric constant of the selected substrate material for a transmission line, passive RF device, or radiating element determines the physical wavelength of RF energy at a given frequency for that structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is reasonably suitable for all of the various passive components, radiating elements and transmission line circuits to be formed on the board.
In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to have a length of a quarter wavelength. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide for practical implementation. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line or a radiator element can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. This could be accomplished by selecting a board material with a high relative permittivity, such as 50 to 100. However, the use of a dielectric with a high relative permittivity will generally result in a significant reduction in the radiation efficiency of the antenna.
Antenna elements are sometimes configured as microstrip antennas. Microstrip antennas are useful antennas since they generally require less space and are generally simpler and are generally less expensive to manufacture as compared to other antenna types. In addition, importantly, microstrip antennas are highly compatible with printed-circuit technology.
One factor in constructing a high efficiency microstrip antenna is minimizing power loss, which may be caused by several factors including dielectric loss. Dielectric loss is generally due to the imperfect behavior of bound charges, and exists whenever a dielectric material is placed in a time varying electrical field. Dielectric loss generally increases with operating frequency.
The extent of dielectric loss for a particular microstrip antenna is primarily determined by the dielectric constant of the dielectric space between the radiator patch and the ground plane for a patch antenna having a single patch. Free space, or air for most purposes, has a relative dielectric constant approximately equal to one.
A dielectric material having a relative dielectric constant close to one is considered a “good” dielectric material. A good dielectric material exhibits low dielectric loss at the operating frequency of interest. When a dielectric material having a relative dielectric constant substantially equal to the surrounding materials is used, the dielectric loss is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip antenna system involves the use of a material having a low dielectric constant in the dielectric space between the radiator patch and the ground plane.
Furthermore, the use of a material with a low relative dielectric constant permits the use of wider transmission lines that, in turn, reduces conductor losses and further improves the radiation efficiency of the microstrip antenna. However, the use of a dielectric material having a low dielectric constant can present certain disadvantages.
One typical disadvantage is that it is difficult to produce high-speed compact patch antennas spaced from a ground plane using a low dielectric constant dielectric. When a dielectric material having a low relative dielectric constant (such as 1-4) is disposed between a patch and a ground plane, the resulting patch size is large, sometimes large enough to preclude use in a given application, such as in some RF communication systems.
Another problem with microstrip antennas is that the feed efficiency often degrades substantially as the patch is spaced further away from the ground plane. That said, more spacing of the patch from the ground plane is also advantageous and, as such, is usually accommodated using dielectric material with a higher dielectric constant to fill the space between the patch and the ground plane. Unfortunately, efficiency is generally substantially compromised in order to meet other design parameters.